Push-pull circuit with driving assisted by asymmetric charge sharing

1. A push-pull circuit for an opto-electronic device, comprising: an output node connected to a common node, wherein an input signal is transmitted over a line from the common node to the output node; a pull-up circuit connected to the output node that, in operation, controls a falling edge rate of the input signal to the opto-electronic device while sharing charge with the output node, the pull-up circuit comprising: a current source; a variable capacitance capacitor in parallel with the current source; and a controllable switch in series with the current source and the variable capacitance capacitor to connect and disconnect the current source and the variable capacitance capacitor to and from the output node; and a pull-down circuit connected to the output node that, in operation, controls a rising edge rate of the input signal to the opto-electronic device while sharing charge with the output node, the pull-down circuit comprising: a current sink; a variable capacitance capacitor in parallel with the current sink; and a controllable switch in series with the current sink and the variable capacitance capacitor to connect and disconnect the current sink and the variable capacitance capacitor to and from the output node, wherein the pull-up circuit and the pull-down circuit is configured to convert the input signal at the output node to an output signal to the opto-electronic device that equalizes the output signal of the electro-optic device based on a difference in the falling edge rate of the input signal caused by the pull-up circuit and the rising edge rate of the input signal by the pull-down circuit.

2. An apparatus comprising: an opto-electronic device; a driver circuit that, in operation, asymmetrically drives an input signal to the opto-electronic device to equalize output from the electro-optic device, the driver circuit including: an output node connected to a common node, wherein the input signal is transmitted over a line from the common node to the output node and the input signal is transmitted to the opto-electronic device through the output node; a pull-up circuit connected to the output node including: a current source; a first variable capacitance capacitor in parallel with the current source; and a first controllable switch in series with the current source and the first variable capacitance capacitor to connect and disconnect the current source and the first variable capacitance capacitor to and from the output node, the pull-up circuit sharing charge between the output node and the first variable capacitance capacitor to control the falling edge rate of the input signal to the opto-electronic device when connected to the output node; and a pull-down circuit connected to the output node including: a current sink; a second variable capacitance capacitor in parallel with the current sink; and a second controllable switch in series with the current sink and the second variable capacitance capacitor to connect and disconnect the current sink and the second variable capacitance capacitor to and from the output node, the pull-down circuit sharing charge between the output node and the second variable capacitance capacitor to control the rising edge rate of the input signal to the opto-electronic device when connected to the output node, wherein the driver circuit asymmetrically drives the input signal based on a difference in the falling edge rate of the input signal caused by the pull-up circuit and the rising edge rate of the input signal by the pull-down circuit at the output node to output an output signal that asymmetrically drives the input signal to the electro-optic device.

3. The apparatus of claim 2 , wherein the opto-electronic device is a vertical-cavity surface-emitting laser.

4. The apparatus of claim 2 , wherein the driver circuit is a link optical driver.

5. The apparatus of claim 2 , wherein at least one of the first variable capacitance capacitor or the second variable capacitance capacitor further comprises: a plurality of capacitors arranged in parallel; and a plurality of controllable switches, each controllable switch of the plurality of controllable switches being arranged in series with a respective one of the plurality of capacitors and switching the respective one of the plurality of capacitors in and out of the pull-down circuit or the pull-up circuit.

6. The apparatus of claim 5 , wherein the plurality of switches are controlled by a programmable signal.

7. The apparatus of claim 6 , further comprising means for programming the programmable signal.

8. A method comprising: generating an input signal for an opto-electronic device from a push-pull driver circuit; sharing charge in the push-pull driver circuit between the input signal and at least one variable capacitance capacitor to control at least one of the rising edge rate or the falling edge rate of the input signal, wherein each of the at least one variable capacitance capacitors is connected to an output node, the output node connected to a source of the input signal; and asymmetrically driving the opto-electronic device with the controlled-edge input signal to produce an equalized optical output signal from the opto-electronic device, wherein a first variable capacitance capacitor of a push portion of the push-pull driver circuit is in parallel with a current source, and a second variable capacitance capacitor of a pull portion of the push-pull driver circuit is in parallel with a current sink, and wherein asymmetrically driving the opto-electronic device with the controlled-edge input signal is based on a difference in the falling edge rate of the input signal and the rising edge rate of the input signal on the output node to output an output signal that equalizes the output of the opto-electric device.

9. The method of claim 8 , wherein sharing charge in the push-pull driver circuit includes: sharing charge between the input signal and the first variable capacitance capacitor in the push portion of the push-pull driver circuit; and sharing charge between the input signal and the second variable capacitance capacitor in the pull portion of the push-pull driver circuit.

10. The method of claim 9 , wherein at least one of sharing charge between the input signal and a first variable capacitance and sharing charge between the input signal and the second variable capacitance includes programming a signal to control the variable capacitance of at least one of the first variable capacitance and the second variable capacitance.

11. The method of claim 8 , wherein sharing charge in the push-pull driver circuit between the input signal and at least one variable capacitance capacitor to control at least one of the rising edge rate or the falling edge rate of the input signal comprises sharing charge between the input signal at the first variable capacitance capacitor in a pull-up circuit to control the falling edge rate of the input signal.

12. The method of claim 8 , wherein sharing charge in the push-pull driver circuit between the input signal and at least one variable capacitance capacitor to control at least one of the rising edge rate or the falling edge rate of the input signal comprises sharing charge between the input signal at the second variable capacitance capacitor in a pull-down circuit to control the rising edge rate of the input signal.

13. The method of claim 8 , wherein the at least one variable capacitance capacitor comprises a plurality of capacitors in parallel and the value of the capacitance is controlled by switching the capacitors in and out of the push-pull driver circuit.

14. The method of claim 8 , wherein the opto-electronic device comprises a vertical-cavity surface-emitting laser and the push-pull driver circuit comprises a portion of a link optical driver.